Pollutants in water have significant impact on human health and the natural environment. Environmental contamination, such as high levels of nutrients, industrial wastes, toxic chemicals, and algal blooms can lead to mass mortality in fish and seabirds and may possibly result in disease outbreaks. Conventional water quality evaluation is typically conducted by on-site sampling followed by transport to a laboratory for testing, or on-site data collection. Such procedures are costly, time consuming, and require skilled operators. Further, the test results can only indicate the quality of water at the specific time and location of sampling. Water quality preferably should be monitored in a fast and efficient manner and in real time which allows the responses to be used to adequately address the sources of water contamination as quickly as possible. Recent developments have shown a trend toward continuous data collection using in situ detectors [1].
Carbon nanotubes have a high aspect ratio, large surface area, and unique electrical properties that offer great potential in chemical and biological sensing applications [2, 3]. Nanotube based sensors exhibit fast response (less than 5 seconds in response to pH buffers [4]), high sensitivity (down to 20 ppb in response to dimethyl methylphosphonate in water [5]) and are miniature in size. As an essentially one-dimensional nanomaterial composed of a single atomic layer, single-walled carbon nanotubes (SWNT) are extremely sensitive to chemical and environmental conditions, and the conductance of SWNTs can change dramatically when exposed to a low concentration of ions or molecules in liquid. Various SWNT sensors have been developed and utilized for liquid analysis, including sensors for pH value [4], heavy metal ions [6], toxic organics [5], bacteria [7], and viruses [8].
Microfluidics technology is being increasingly applied in chemical, biological and medical diagnostics of solution-based samples. Microfluidics offers numerous attractive features, such as the ability to use small amounts of samples or reagents; to carry out sample separations and detections with high resolution and sensitivity; to significantly reduce the cost per analysis; to replace batch analysis with continuous flow analysis; and to reduce the footprint of analytical devices [9]. Microfluidic systems are able to manipulate and examine samples containing a single cell or a single molecule [10], which is especially important for bioanalysis. Use of droplet fluidics is a new trend in microfluidics systems, which includes the control of the droplet volume, chemical concentrations within the droplet, and sorting of droplets based on flow pattern [11].
Integration of an SWNT sensor with a microfluidic system would enable the development of a lab-on-a-chip device that can perform water quality monitoring and other types of analysis of liquid samples. Such a chip would replace many types of measurement that are normally performed manually in a lab using bulky equipment. With a suitable design, the microfluidic system can carry out sample preparation steps, including filtering and separating various components in a liquid sample, and then guide the solution to a nanosensor array for analysis.
In order to develop a highly sensitive and autonomous microdevice for real-time in situ water quality monitoring, the SWNT sensors have to be integrated with the microfluidic system, e.g., on a single chip. Permanent bonding of a microfluidic channel onto a silicon substrate requires treatment with an oxygen plasma. However, such a plasma treatment of a carbon nanotube-based device would damage the nanotubes. Fu and colleagues integrated a SWNT sensor with a microfluidic channel by covering the SWNT with a continuous metal layer which protected the nanotubes underneath during exposure to oxygen plasma [12]. After bonding the SWNT device with a microfluidic channel made of polydimethylsiloxane (PDMS), an etching solution was introduced through the channel to remove the metal mask layer covering the SWNT, so that the SWNT inside the microfluidic channel could be used for sensing applications [12]. However, this type of integration process could introduce contamination onto the SWNT from the etching solution and the substances generated by the chemical reactions. Bourlon and colleagues have fabricated a flow and ionic sensor using a nanotube transistor covered with a PDMS channel without using an oxygen plasma treatment [13]; however, the microfluidic channel was not well sealed on the device which resulted in leakage of solution. Thus, there remains a need to develop procedures and devices that incorporate SWNT into microfluidic channels without compromising the integrity of the SWNT or the fluid handling of the microfluidics system.